Many industries and products process semiconductors and semiconductor wafers into other products, such as microprocessors, computer memory, solar cells, and other devices. Semiconductor wafer have become a commodity in many cases, but the process of making wafers is still largely inefficient.
In the conventional wafering process, multi-crystalline (mc) and single-crystalline (sc) Silicon (Si) wafers are obtained by unidirectional slicing of Silicon ingots. Such processes currently account for more than 90% of annual solar cell production, and are one of the most important technologies considered for the developing photovoltaic industries today. Making solar cells from single and multi-crystalline ingots of high purity Silicon is energy and capital intensive. The process is difficult to scale up due to the batch nature of many of the processing steps and the wafer process is inefficient. The wires used to cut the slabs into wafers are as thick as the wafers themselves and half of the material is therefore lost in the sawing process. This cutting material is difficult to recycle since it is mixed with iron from the wire and cutting oil. Fracture behavior, crack density, thickness variation, surface roughness and cleanness are some of the major determinants for the wafer quality with respect to sawing process. These issues make it difficult and inefficient to make very thin wafers. The material usage is therefore much higher than needed.
These deficiencies are particularly problematic in the production of solar cells where production cost is one factor which limits widespread acceptance. The rapid growth of the photovoltaic industry has put significant pressure on the industry to develop new processes for making solar grade Silicon and new technology for making solar cell wafers. During the last decade the industry growth has been 25-35% per year despite the high cost of solar generated electricity (about 4 times higher than electricity generated by a pulverized coal fired power plant.). Therefore, there is a need for new technology that will reduce cost and allow faster scale-up.
There have been several prior art approaches to reduce the cost of producing silicon wafers. One type of approach has been to abandon the batch fabrication process that dominates the industry in favor of a continuous film manufacturing process. Some of the continuous film processes on the market are Edge-defined Film-fed Growth (EFG) from Schott Solar, String Ribbon (SR) from Evergreen Solar, Molded Wafer (MW) from GE Energy (formerly known as Silicon Film™, Astro-Power), Ribbon-Growth-on-Substrate (RGS) from ECN and Crystalline-Silicon on Glass from CSG Solar.
All of these processes have one or more drawbacks. There are primarily three major processes that involve low-, intermediate- and high-temperature solid support materials. Interaction of these with Silicon influences the growth conditions of Silicon thin layer thereby affecting the crystallographic, optical and electrical properties. The Molded Wafer (MW, η=11-14%) and Ribbon Growth on Substrate (RGS, η=13-14%) processes involve solid substrates or support materials. The MW process [Hall R B, Barnett A M, Collins S R, Checchi, J C, Ford D H, Kendall C L, Lampo S M and Rand J A. Columnar-grained polycrystalline solar cell and process of manufacture. U.S. Pat. No. Re. 36,156 Mar. 23, 1999; and Grenko A, Jonczyk and Rand J. Single wafer casting. In: IEEE 4th World Conference on Photovoltaic Energy Conversion. 1415-1417 (2006).] makes wafers by melting Silicon powder in an IR furnace on continuously moving sheets of Silicon nitride coated ceramics. In the back end of the furnace the melt is cooled, similar to the Pilkington process, except that the substrate in the MW process is solid rather than liquid. However, the solid SiNx provides a large number of nucleation sites and the Silicon sheet is multi-crystalline with columnar crystal in the sub-mm to μm range. The RGS process [Seren S, Hahn G, Gutjahr A, Burgers A R, Schöecker A, Grenko A, Jonczyk R. Ribbon Growth on Substrate and molded wafer—two low cost silicon ribbon material for PV. In: IEEE 4′th World Conference on Photovoltaic Energy Conversion, 1330-1333 (2006).] uses a moving substrate (graphite or ceramic) underneath a shaping die filled with molten Silicon, where rapid crystal growth and direct contact with the cold, solid substrate material produces a sheet with crystal grains similar to the MW process. High crystal defect density (dislocation density, grain boundaries) and impurities reduce the efficiencies of the MW and RGS processes [Seren S, Hahn G, Gutjahr A, Burgers A R, Schöecker A, Grenko A, Jonczyk R. Ribbon Growth on Substrate and molded wafer—two low cost silicon ribbon material for PV. In: IEEE 4th World Conference on Photovoltaic Energy Conversion, 1330-1333 (2006).]. High grade borosilicate/soda-lime glasses (3-5 mm thick) have also been employed commercially as a substrate for the production of Crystalline Silicon on Glass (CSG, η=7-9%) thin-film solar cells. The process utilizes polycrystalline Silicon (pc-Si, (epitaxial growth, grain size >20 μm)) as a seed layer. This cell eliminates the need for transparent conducting oxide together with the associated cost and the problems associated with stability of amorphous Silicon. Low material (Si) use, large monolithic construction, durability and ruggedness are the advantages of this type of solar cell with conversion efficiency in the range of 7-9% [Liu F, Romero M J, Jones, K M, Norman A G, Al-Jassim, M M. Intragrain defects in polycrystalline silicon thin-film solar cells on glass by aluminum induced crystallization and subsequent epitaxy. Thin Solid Films, 516, 6409-6412 (2008) and literatures thereof.]. Due to the polycrystalline nature of the seed layer, heavily defective grains always remain present in the epitaxial layer. Intra-grain defect is a major limiting factor for the electrical quality of pc-Si layer [Green M A, Basore P A, Chang N, et. al Crystalline thin-film solar cell modules. Solar Energy, 77, 857-863 (2004).].
There are some methods disclosed in prior art describing a continuous process of producing silicon wafers on molten support materials such as Tin (Sn) and Lead (Pb), similar to the float glass process. By adjusting the composition of molten support materials, spreading of Silicon and desired equilibrium thickness of silicon film can be achieved in this way. As observed in the float glass process, defects arise when the voltatilized materials from the molten surface are advected and diffused in the gas flow followed by their dropping on the top surface glass.
Glasses are known for many practical applications. Use of borosilicate and aluminosilicate glasses as substrate materials for semiconductor and photovoltaic devices are disclosed in prior art. Various methods are directed to deposit thin-film on glass substrate. These polysilicon films are limited by the non-uniformity and low electron mobility.
Accordingly, there is a need for improved silicon wafers or sheet production, particularly for low cost processes that scale up and allow for the production of textured high quality silicon multi-crystalline or single crystalline structures. Those and other advantages of the present invention will be described in more detail herein below.